MRAM has tremendous potential as a nonvolatile, solid-state memory to replace flash memory and electronically erasable programmable read-only memory (EEPROM). To enhance the performance of MRAM chips, it is necessary to reduce the lithographic minimum printable size. However, ion beam etching (IBE), a.k.a. ion milling, has been essentially the only method available for creating fine patterns, e.g., submicron patterns, in magnetic thin film structures. Because of the lack of volatile compounds for ferrous metals other than carbonyl, reactive-ion etching (RIE) has not been a viable technique for patterning thin magnetic films; and a RIE process based on carbonyl chemistry has not yet been developed. Thus, a chemical etching technique for patterning magnetic thin films based on Fe, Co and Ni is attractive because of the thin film nature of MRAM magnetic films (20-50 Å z-direction) relative to the x-y dimensions of patterned magnetic tunnel junction (MTJ) elements. The MRAM structure represents a complex multilayer system which includes numerous magnetic thin film layers. A typical MRAM structure is shown in FIG. 1. Specifically, the thin film structure shown in FIG. 1 comprises Si substrate 10, SiOx layer 12, a 150 Å Ti layer 14, Ni81Fe19 (40 Å) layer 16, Ir20Mn80 (120 Å) layer 18, CO90Fe10 (20 Å) layer 20, Al2O3 (10 Å) layer 22, Ni81Fe19 (40 Å) layer 24 and Ti (100 Å) layer 26. In this prior art magnetic structure, Al2O3 layer 22 serves as a tunnel barrier between the top magnetic film layer, i.e., Ni81Fe19 layer 24, and the pinned bottom magnetic layer, i.e., Co90Fe10 layer 20, the antiferromagnetic layer 18 and the magnetic layer 16 which are present beneath the tunnel barrier layer 22. Layer 26 is a passivating layer that prevents moisture, air or other contaminants from entering into the structure, while layer 14 is an adhesion layer. In the case of AP-pinned MTJs, the antiferromagnetic layer 18 can be a Ru spacer. The term “AP-pinned MTJs” is used herein to denote MTJs which contain an antiparallel-pinned (AP) layer structure, wherein the AP layer structure includes at least two pinned layers having magnetic moments that are self-pinned antiparallel to each other and the at least two pinned layers are separated by an AP coupling layer or a spacer.
As is well known to those skilled in the art, the magnetic films of a MRAM structure, such as illustrated in FIG. 1, are quite thin. Patterning of prior art MRAM structures, such as shown in FIG. 1, is typically carried out by first applying a mask to the MRAM structure and patterning the mask by lithography (exposure and development). FIG. 2 shows the structure after these steps wherein reference number 28 represents the patterned mask. The pattern is transferred to the MRAM structure by RIE, IBE, or wet etching. And then the exposed Ni81Fe19 (40 Å) layer 24 can be pattern-wise etched by RIE, IBE, or wet etching. In a traditional wet etching process, a standard aqueous acid solution, such as sulfuric and/or nitric acid, is employed to etch the exposed Ni81Fe19 (40 Å) layer 24. Although the acid etchants are capable of etching through the exposed top magnetic layer 24 of the structure, the acid etchants are not selective for removing just that magnetic layer 24. Instead, when the acid etchants are employed, they also etch the underlying alumina tunnel barrier layer 22, the pinned Co90Fe10 layer 20, and the Mn in the Ir20Mn80 layer 18 of the magnetic thin film stack providing the structure shown in FIG. 3.
Despite being capable of etching numerous magnetic layers in a MRAM structure, the use of prior art aqueous acid solutions causes Galvanic-coupling-accelerated dissolution of the CO90Fe10 (20 Å) layer 20 which is coupled to an antiferromagnetic layer in simple-pinned MTJs and to a Ru spacer in AP-pinned MTJs. The term “simple-pinned MTJs” is used herein to denote MTJs which contain a single reference layer, wherein the single reference layer has its magnetization typically pinned by exchange coupling with an antiferromagnetic layer. In MRAM, due to coupled active magnetic layers and noble metals, avoidance of Galvanically-accelerated dissolution is a major concern.
U.S. Pat. No. 6,426,012 describes a method to pattern the magnetic soft layer of a MRAM structure while avoiding Galvanically-enhanced etching reactions. However, the carboxylic acids employed in U.S. Pat. No. 6,426,012 are weak acids, and thus are not capable of etching through the alumina tunnel barrier 22.
A desirable situation would be to selectively etch through the exposed top magnetic layer, i.e., layer 24, as well as the underlying thin Al2O3 layer 22, and the pinned bottom magnetic layer, i.e., layer 20, in the MRAM structure, and stop the etch process at the antiferromagnetic layer 18, while avoiding Galvanically-enhanced etching reactions. In the case of AP-pinned MTJs, the etching process stops at the Ru spacer of the AP-pinned layer structure. Such a method would leave the antiferromagnetic layer, in the case of simple-pinned MTJs, and the Ru spacer, in the case of AP-pinned MTJs, unetched.
To date, applicants are unaware of any etching process which selectively etches a magnetic thin film structure so as to stop on the antiferromagnetic layer, in the case of simple-pinned MTJs, or on the Ru spacer, in the case of AP-pinned MTJs, while avoiding Galvanic corrosion or Galvanically-assisted lateral etching of the edges of exposed magnetic layers. There is thus a need for developing an etching process which is capable of selectively etching a magnetic thin film structure to provide a patterned structure wherein the pattern is not formed in the antiferromagnetic layer, in the case of simple-pinned MTJs, or in the Ru spacer, in the case of AP-pinned MTJs. Such an etching process would be beneficial since it would prevent unwanted Galvanic corrosion of the inner magnetic layers, while being able to pattern the top magnetic film layer, the tunnel barrier layer, and the pinned bottom magnetic layer of the structure. The ability to etch down to an etch stop layer such as the antiferromagnetic layer or the Ru spacer further has advantages of superior process repeatability and avoiding the difficulties associated with stopping the etch process at alumina and leaving intact the pinned magnetic layer.